Photographic objective



Jan. 23, 1934. H. s. NEWCOMER I PHOTOGRAPHIC OBJECTIVE 2 Sheets-Sheet 1Original Filed May 18, 1932 ATTORNEYS H7 4 v 3 m J 2 a w .wiw x 4 H W 6i w 2f; W r Q\W, Zfl a a 4 m 0 n! 6 4 Jan. 23, 1934.

H. s. NEWQOMER PHOTOGRAPHIC OBJECTIVE 1 2 Sh'eets-Sheef 2 Original FiledMay 18, 1952 Reissuecl Jan. 23, 1934 UNITED STATES 19,056 PHOTOGRAPH 1CQBJECTIVE Harry Sidney Newcomer, New York, N. Y.

Original No. 1,932,082, dated October 24, 1933, Serial No. 611,964, May18, 1932. Application for reissue December 1,

1933. Serial No;

Claims; (c1. 88-57) This invention relates to photographic (orprojection) optical systems of the type in which an anamorphoser isincluded as a part of the system and has for an object to provide animproved 5 spherical lens for use between the anamorphorser and theobject (or image) as a correcting lens to cause rays from the object totraverse the anamorphoser as pencils of parallel rays.

Anamorphosing objectives of the type used in optical systems as aboveindicated may be of two kinds, cylindrical objectivescomprising suitablydesigned and suitably spaced cylindrical elements with their axes allparallel, and prism objectives comprising two prisms with their basesoppositely arranged, the median planes of the prisms forming a V. Themagnification due to the anamorphoser is produced in a so-called activeplane, a plane perpendicular, in the first case to the axes of thecylindrical elements and in the second to the two planes of the V.

Improved anamorphosers of both types have been discussed in myco-pendingapplications for Letters Patent.

The general theory of such anamorphosing objectives haslbeen discussedby Rudolf in the British Patent No. 8512 of 1898. As is there disclosed,when neither the image nor object is at a great distance from theanamorphoser it is desirable to place the anamorphoser between twospherical lens systems each having the focal length of the correspondingimage distance. Since one image distance is usually much less than theother the lens on the former side has the relatively greater opening andis usually a suitably corrected ordinaryphotographic or projectionobjective. The lens on the side of the longerimage' distance is the one,designated herein as the correcting lens. k v t r In the case ofcylindrical anamorphosing objectives comprising cylindrical elementswith their axes all parallel the necessity of such a correcting lens maybe obviated by a focussing arrangement permitting the approximation ofthe elements when the object (as in photography) is nearer thaninfinity- There are, however, certainpractical advantages in the use ofa correcting lens even with an anamorphoser of this type.

I In the case of the prism anamorphoser such a 0 correcting lens is forall ordinary object distances practically a necessity. Such a lens, bymaking paraxial pencils traversing the prism anamorphoser pencils ofparallel light, obviates the introduction of astigmatism which wouldresult 5 from the refraction of divergent or convergent certain featuresof the invention,

. tain features of the invention,

pencils at the plane surfaces of the prisms; Thus, forobjects (orimages) at a lesser distance than infinity the refraction at obliqueincidence on the plane refracting surfaces of the prisms of divergent orconvergent pencils produces astigmatism of the pencils. This astigmatismincreases with increasing angles of incidence and also with the nearnessof the object (or image), 1. e. with the increasing divergence orconvergence of -'the pencils. v

Correcting lenses as heretofore used with anamorphose'rs'have beendesigned to correct the astigmatism of paraxial rays but have notsatisfactorily corrected the astigmatism of diagonal rays with resultantpoor definition at the margin of the image.

The present invention provides a correcting lens of such design thatastigmatism arising in the system or at the correcting lens alongdiagonal rays. that is to say along incident pencils 75 coming fromnon-axial, points of the object, is more effectively avoided g Thenature and objects of the invention will be better understood from adescription of selected illustrative embodiments thereof and anexplanation of the principles involved for the purpose ofwhichdescription and explanation reference should be had to the accompanyingdrawings forming a part hereof and in which:

Figure 1 is a diagrammatic representation in longitudinal section of anoptical system comprising a photographic objective, a cylindricalanamorphosing objective and a correcting lens embodying certain featuresof the invention;

Figure 2 is a diagrammatic representation in longitudinal section of anoptical system comprising a photographic objective, an anamorphosingprism objective and a correcting lens embodying Figure 3 1s adiagrammatic representation in longitudinal section perpendicular to theactive plane of an anamorphosing prism objective and an achromaticcorrecting'lens embodying cerj- Figure! shows graphs indicating thecurvature in diopters of the front surface for correcting lenses of Adifferent thicknesses having varying strengths in diopters and certainother special relationships with respect to the associated opticalsystem, whereby oblique homocentric pencils ren main homocentric afterrefraction,

Figure 5 shows a series of simple lenses of the equal strengths but ofdifferent cambrures, i. e.

strengths of front surfaces, together with their 1 .0

distance apart equal to the difference between" their focal distances,that is .with the anterior.

respective anterior image surfaces and focal planes,

Figure 6 is a plot showing deviation of the image surfaces from thefocal plane for a series of such lenses, and

Figure 7 is a similar plot for three other series of certain strengths.

Referring more particularly to the drawings there are illustrated notonly anamorphosing optical systems in which there are provided sphericalcorrecting lenses which when. associated with suitable optical systemsare designed in each case to give the least average astigmatism for thewhole area of the image, but also certain relationshipsassociated'withthe design of such a lens. The principles involved in thedetermination of the optimum cambrure of such lenses will be betterunderstood from a consideration of the illustrative examples.

The correcting lens is associated with an anamorphoser and aphotographic or projection objective. For convenience the anamorphosermay be considered to have, as in some cases it does have, a flat field.The photographic or projection objective rarely has a fiat field, itstangential and sagittal focal surfaces curve in varying fashion out ofthe focal plane. To simplify the problem we will consider thephotographic objective to have for the moment a fiat field. Then if thecorrecting lens has a flat field, the marginal definition of the picturewill not be interfered with by virtue of oblique astigmatism or failureto focus in the image plane.

The image surfaces of simple correcting lenses are in general far frombeing either fiat or coincident. I have discovered that by choosing asuitable cambrure for the'correcting lens suitable curvatures of itsimage fields may be obtained and the marginal imagery can be substantially improved.

Astigmatism of oblique rays introduced by correcting lenses increasesonly moderately with moderate variations of. the cambrure of thecorrecting lens from the optimum but for most curvatures chosen atrandom the astigmatism thus introduced is so great asto cast a decidedlyinferior definition-at the margin.

The illustrative arrangement of-Fig'. 1, comprises an ordinaryphotographic (or projection) objective 1 with the stop. at the position2, an anamorphoser consisting of the positive cylindrical member 3 andthe negative cylindrical member 4. In front of the anamorphoser is acorrecting lens 5 designed in accordance withcertain principles of thepresent invention.

The cylindrical members 3 and 4 are spaced a focal .point of thepositive member coinciding with the posterior focal point of thenegative member, whereby paraxial pencils of parallel rays incident ontheanamorphoser exit as pencils of parallel rays. r

The correcting lens 5 is a spherical meniscus lens concave toward theanamorphoser, its focal length being equal to the object distance(anterior focal distance equal to the object distance). It follows thatthe. anterior focal plane 6, of the lens is in the plane of the object.The front and back radii of the lens 5 are 34.4 mm. and 66.2 mm.respectively, its thickness 3.7 mm., index of refraction 1.52 and itsstrength is about 7.5 diopters.

The strength of the correcting lens should be such that its anteriorfocal distance is approximately equal to the distance from thecorrecting lens to the object plane (or the average object distance ifthere is variation), in order that the pencils of rays may pass throughthe anamorphoser as pencils of substantially parallel rays. Variationsof diopter from the true distance seem to be tolerated in motion pictureprojection. In the projection of financial news at least twice this muchis tolerated. The resulting astigmatism introduced by the anamorphoseris of course much less than this amount. The focus of the whole systemis likewise thrown out by much less than this. For a mean focuscorrection or adjustment is made by shifting the ordinary photographicobjective.

At 7, Fig. 1, I show an approximately homocentric pencil obliquelyincident on the optical system 5, 4, 3, 1 and coming from points 9, 10,of the focal surfaces T17, 518 of the lens 5, near the margin of itsimage field. The pencil 7 on refraction at'the lens- 5 becomes a pencilof parallel rays. If prolonged as the pencil 11,

its central ray intercepts the axis of the lens 5,

and of the system, in the point 12, and at an angle, for this example,of about 21. The distance from the point 12 to the adjacent surface ofthe lens 5 may be called the posterior intercept or intercept distancefor inclined pencils of parallel rays, and may conveniently be used as afactor of reference for the optimum form of, the correcting lens 5. Inthis illustrative example it is 23 mm. The point 12 locates the'positionof the natural diaphragm of the lens 5. The pencil '7 is refracted bythe,cylindrical lenses 4 and 3, its central ray taking the position ofthe ray 13 within the anamorphoser and 14 to the left of thephotographic objective 1 and passing through the center of the stop ofthe photographic objective at the point 2. The paraxial pencil 15 is ofcourse homocentric. The pencil '7 as drawn is approximately homocentric.This pencil could readily have been made absolutely homocentric if inview of other factors involved this had been, as it might have been, anadvantage. The pencil 15 converges to the point 16 in the object orfocal plane 6. The oblique pencil 11, being by definition a pencil ofparallel rays, is homocentric and on refraction at the lens 5 it becomesthe pencil 7 whose character depends on the intercept distance 5 to 12,the strength of the lens 5 and its cambrure. For certain relationshipsof these factors the pencil 7 is homocentric and then converges to apoint on coincident anterior focal surfaces. Such coincident surfacesare concave toward the lens and the anastigmatic focal point lies at acertain distance from the focal plane. In the illustrative example a 7diopter lens with 23 mm. intercept and 21 angle such distances from thefocal plane would be about 6 mm. corresponding to a focusing error ofabout 0.31 diopters. Such a curvature might for some purposesbe'unobjectionable and the total freedom from astigmatism of obliquerays desirable. In the case of longer focus correcting lenses such asare more frequently cmployed, the curvature of field expressed indiopters is absolutely and frequently relatively less.

In the illustrative example, Fig. 1, at the expense of a certain lack ofhomocentricity (coincidence of anterior focal points) the field has beenflattened as much as possible. The cambrure of the lens 5 is chosen soas to produce a tangential focal surface T17 which nearly coincides withthe focal plane 6. The sagittal focal the correcting lens, the sagittalfocal point of 'of the prisms 23 and 24 and in front of which thecorrecting lens'25 is placed.

The lens 25 is shown as a spherical meniscus lens of 0.2 diopter, theradii of curvature of the front and back surfaces of which are 400 mm.and

468 mm. respectively. In this Fig. 2, I have shown the members of theanamorphosing prism obwhose central one is the ray 33.

bundle of parallel rays.

jective traversed by a bundle of parallel rays The bundle of parallellight 33, refracted at the various surfaces, is convergent in the point2'7 of the focal plane 26 of the photographic objective 21 and at theextreme right it is homocentric and convergent in the point 40 on theanterior focal surfaces of the lens 25. Its central ray, indicated at37, after refraction by the lens 25, when prolonged, intercepts the axisof the lens 25 in the point 32, this pencil here constituting a Theintercept distance in this illustrative example is 125 mm. The axis 35on the expansion side, is displaced somewhat downward, toward the V ofthe anamorphosing prism objective, so as to be no longer in line withbut nevertheless still parallel to the axis of the photographicobjective 21. This lateral displacement of the axis on the expansionside is an inherent characteristic of prism anamorphosers. The pencil 37is inclined at an appreciable angle to the axis of the systems, imaginga point 40 toward the margin of the object. The pencil is shownforeshortened in the drawings.

The anterior tangential and sagittal focal surfaces of the lens 25 areof substantially the same curvature and they are coincident at the point40. This point is about 0.007 diopter short of the frontal plane passingthrough the axial focal point of the lens; By bending the lens so as tosubstantially increase the strength of its front surface (by aboutdiopter) the tangential In Fig. 3, I have image point could be pushedforward to the plane and the sagittal image to within about 0.005

diopter of the plane. As much more increasein cambrure would push thetangential image surface as much beyond the plane as the sagittalsurface would be short of it, about 0.004 diopters.

See Fig. '7.

shown diagrammatically a similar arrangement except that thephotographic objective is only shown schematically by the position ofthe center of its stop at 42 and also the longitudinal section is takenperpendicular to the active plane-instead of parallel to it as in Figs.1 and 2. The prism system 43, 44 com prises two simple thin prisms shownin rectangular section. The V of the prism ariamorphoser is in front (orbehind) the plane of the paper.

, At 45, 55, I have shown an achromatic meniscus correcting lenscomprising a flint glass 45, and a crown glass 55 cemented together. ofthis lens is A diopter.

The strength The radii of curvature of the surfaces, front to back, andthe thick nesses and indices of the glasses are The inclined pencil 47is homocentric and after refraction at thelens 45, 55 it is a bundle ofparallel rays 53 traversing the prisms 44, 43 and are similar to thatdescribed for the illustrative example of Fig. 2, and could be modifiedin the same manner.

In Figs. 1 and 2, the magnification due to the anamorphoser asillustrated is about 50% (121.5) and hence in the active plane, (thesections shown) the rays inclined to the axis and passing through thecenter of the .stop of the photographic objective, when prolongedbackward from the posterior surface of the correcting lens, cut the axisin a point at a less distance from the stop than the distance of thestop. The intercept distance is less than the stop distance, in fact itis equal to the reduced stop distance divided by the magnificationratio, in this instance 3/2. The reduced distance between any two pointsis equal to the sum of the distances in the respective-media eachdivided by the corresponding index of refraction. The magnificationratio is, by definition, equal to the ratio of the dimensions of -theanamorphosed to the non-anamorphosed image. If the glasses of theanamorphose'r have zero thickness then the stop and intercept distancesare proportionate to the magnification. Correction for thickness is madeby substitutingreduced distances for actual distances. InFig. 1, thedistance of the point 12 from the lens 5 is a reduced distance as" thepresence of the lens 4 is ignored so far as the pencil 11 is concerned.

Figures 1 and 2 primarily illustrate the conditions to be consideredwhen designing correcting lenses where the correction of oblique pencilsin the active plane is more important than in the inactive plane, forinstance because the angular field is greater.

The oblique astigmatism and the distances of the curved focal surfacesfrom the focal plane of a simple correcting lens in general increasewith the amount of the .obliquity approximately as the square of theangle. If the astigmatism or other field curvature defect be annulledfor any given angle then it will not be zero at lesser or greater anglesbut the-deviation from zero is less for lesser angles. It is thereforeusually advisable to correct for the maximum angle if an optimumcorrection is desired.

In Fig. 3, the situation in the inactive plane of the anamorphoser isdepicted. The intercept distance is equal to the reduced stop distance.

In some projecting systems the image fields have much greater angularextension in the inactive plane than the active plane and hence thecorrection for inclined pencils in'the inactive plane is more important.such a situation.

An achromatic correcting lens is depicted in Fig. 3, by way ofillustration. It has at the same time certain structural propertiescommon tothe correcting" lenses of Figs. 1 and 2 andpresently to bedescribed.

I have discovered that the marginal portions of the images due tooptical systems of the nature described can be much improved by asuitable choice for the cambrure of the correctinglens.

Figure -3 illustrates The cambrure may be defined as being numerifrontsurface.

faces of a simple lens. As a convenient practical method the cambrure ofa lens of known strength can be determined by fixing the curvature ofthe The optimum curvature depends on the strength of the lens and theintercept distance. To a lesser extent it depends on the thickness ofthe lens, the maximum angle for which it is corrected and the index ofrefraction of the glass. The appropriate first surface curvatures forcorrecting lenses of strength ordinarily employed with anamorphosersbear a fairly constant proportional relationship to the interceptdistance, astigmatism increasing only moderately with slight deviationsfrom the optimum but nevertheless being relatively enormous for mostcurvatures chosen at random. Ordinarily the radius of curvature of thefront surface of the correcting lens is best less than and certainlynever more than ,4; the distance of the object (or image).

When conditions of optimum correction require a substantial flatteningof the tangential image surface of the correcting lens and correspondingreduction in the curvature of its sagittal surface appropriatecurvatures or strengths of the front surface of the lens are greater andwhile the flattening is substantially at an optimum over an appreciablerange of curvatures these curvatures are about the same for all strengthlenses, varying only with the intercept distance.

In Fig. 4 I have shown a graph representing by the curves 61, 62, 63,64, 65, the best curvatures of the front surfaces of correcting lensesof thicknesses 1, 2, 3, 4, 5, mm. respectively and of various strengths0 to 14 diopters corrected for astigmatism of inclined rays, all havingintercept distances equal to 25 mm. (posterior surface of lens to pointof intersection with the axis of inclined pencil of parallel rays). Thecurvatures of the front surfaces of the various lenses "are expressed interms of their strengths in diopters. The range of the graphs is from 4to 15 diopters for strength of the front surfaces. The strength of lenswill depend upon the average objectdistance for which the system is tobe used, the focal length being substantially equal to the objectdistance.

The above graphs 61 to 65 pass smoothly through the following coordinatepoints:

(817,4.0), (9.2,60), (935,80), (10.85,'10.0), (12.1,12.0), (14,133); andI and (40,04), (6.0,.63), (701.6), (7314,30). (81,40), (8.73,6.0)(9.5,8.0), (105,100), (1203,12); and

and

races thickness and intercept distance a proper anastig matic lens canbe taken from the graph, multiplying the data of the appropriate graphlocus by the necessary factor to bring 25 mm. to the required interceptdistance. Thus a 2 diopter lens of mm. intercept distance is the 6diopter lens of the graph with its radii multiplied by 3. The lenses ofthe graph are corrected for an inclination of 20 of the pencil ofparallel rays to the axis. This is an average extreme angle for manypurposes for which these anamorphosers and correcting lenses are used.The rays inclined at a lesser angle than 20 will not be perfectlycorrected but the correction is sufficient to provide entirelysatisfactory results. Lenses perfectly corrected for .a lesser anglehave slightly stronger front surfaces but the difference is notappreciable and the error to be corrected is negligible, that is, theastigmatism of the lenses shown for rays inclined less than 20 isnegligible. For greater angles a perfect correction, the best averagecorrection for all intermediate angles including the maximum, are lenseswith somewhat weaker front surfaces. Again the difference is not great.At 30 the appropriate strength of the front surfaces is roughly onethird diopter less.

At 66 I have drawn a straight line representing the locus of lenseswhose back surface is flat.

Lenses represented by points below this line, i. e., whose back surfacesare convex, have when used in the manner here described so muchastigmaing lenses free from astigmatism along oblique rays. These raysfocus short of the anterior focal plane of the lens. The amounts of suchdeviation from the focal plane has been indicated in a number ofillustrative examples. Attention has also been called to the fact thatdecreasing the cambrure not only introduces an astigmatism of obliquerays, which rapidly becomes considerable in amount, but alsosubstantially increases the curvature of both focal surfaces. Increasingthe cambrure introduces a moderate amount of astigmatism but flattensthe focal surfaces and in many instances reverses the curvature of thetangential focal surface and sometimes of the sagittal focal surface.

In many instances a correcting lens which is substantially anastigmaticfor oblique rays is desirable. Fig. 4 gives the data from which suchlenses may be calculated.

For practical purposes the solution defining tism and curvature of theirmarginal fields as A radius of curvature of the front surface of a.

substantially anastigmatic correcting lens may be taken-as independentof the index of refraction and equal to 2.3 times the distance from thelens to the point at which the inclined refracted ray cuts the axis ofthe lens. This value approximates that given by the data of Fig. 4 for a25 mm. intercept distance lens of 6 diopters strength and all lensesderived therefrom by use of a multiplying factor.

The average front radius factor should be 10% larger or about 2.5

narily in use, as for instance in the projection of financial news. Thefront surface radius factor should be 50% larger for a A; diopter 25 mm.

intercept distance correcting lens. There are obviously variations forthese factors due' to lens thickness variations. l

In Fig. 5 at 71, 72, 73, 74, and 76 I show in longitudinal section aseries of 7 diopter lenses having front surfaces of 7, 9, 12, 14, 17'and 19 diopters respectively. The thickness of lens 71 is 3 mm., of theothers-4 mm. The posterior intercept distances and angles of inclinationof the rays are in each instance 25 mm. and 21.17". At 81, 82, 83, 84,and 86, I show the anterior focal planes and the tangential and sagittalfocal surfaces T and S of these lenses. Lens 74 having a 14 diopterfrontsurface, has the fiattest tangential and sagittal focal surfaces.Either greater or lesser strength of the front surface gives increasedconcavity of both surfaces, that of the tangential focal surfaceincreasing more rapidly than that of the sagittal focal surface withchange in cambrure. In the case of lenses 72 and 76 the surfaces areapproximately coincident. In the case of lens 71 having a flat posteriorsurface the concavity of both surfaces, and particularly of thetangential surface is sufficiently marked to cause a considerablerefractiveerror along oblique rays.

In Fig. 6 I plot the concavity or curvature of these focalsurfaces'with-continuous variations in cambrure, from 7 to 19 dioptersfront surface, the concavities being measured by the distances inmillimeters of the focal points of the 21.17 oblique rays fromthen'focal-plane. Distances up in the figure are measured from the focalplane towards the lens and are represented as having negative values.The posterior intercept distance of the parallel beam is 25 mm. 4

At S7 I plot the sagittal locus and at T7 I plot the tangential locus.The sagittal focal point never reaches the focal plane for any cambrure.The tangential focal point nearly reaches the focal plane over a widerange of cambrures, for instance from 12 to 17 diopters front surface.

All cambrures beginning with values inthe vicinity of the loweranastigmatic value and extending to the upper anastigmatic valuerepresent lens types which when used in combination with one or anotherof the anamorphoser and ordinary objective types make desirable opticalcombinations. Correcting lenses with flat or convex posterior surfacesmake undesirable combinations.

In Fig. 7 I plot to the same scale over a range from 0 to 20 dioptersstrength of front surface. the loci of the distances in millimeters fromthe focal plane of the focal points along 20 oblique rays for lenses ofthree strengths. the loci for a 1 diopter lens. S2 and T2 for a 2diopter lens and S4 and T4'for a 4 diopter lens.

For convenience in identifying pairs or curves l their intersectionpoints.

lens.

S1 and T1 are focal surfaces are convex toward the lens.

relative distances for the various strength lenses are more nearly equalthan the absolute distances shown in the graphs, that is to say, thedistances of the focal points fromthe focal plane would be nearly equalif the focal lengths were reduced to the same scale, for instance unity;

Curves are drawn for a sufficient number of different strength lenses togive a mental picture of the characier of the focal surfaces of possiblecorrecting lenses. Lenses which are strong in relation to interceptdistances (as in Fig. 6) have boih focal surfaces concave towards thelens, but for certain cambrures as shown the tangential focal surface isnearly flat and the sagittal focal surface has a minimum curvature.

When the intercept distance remains constant, as in the graphs, asthe'strength decreases, both image surfaces become less concave, thetangential focal surface being thefirst to become convex toward the.lens. Thus for the 4 diopter lens. Fig. 7, it lies distal to the focalplane for front surfaces 11 to 16 diopters. Lenses of still lessstrength, have convex sagittal as well as tangential focal surfaces forcertain cambrures, (2 diopter lenses, Fig. 7) Still weaker lenses haveagain greater concavity of the focal surfaces toward the lens. Thus the1 diopter lensfof Fig. 7 has, within the useful front surface range, 7to 20 diopters absolutely and relatively greater distances of the,saglttal surface from the focal plane. The maximum convexity of thetangential surface is, as measured in millimeters from the focal surfacealong the oblique rays. not much more than that of the 2 diopter lens.Relatively it is much less. As has already been described the choice ofstrength of lens to be taken from the data lenses of this specificationdepends not only upon the definitive strength of the converted lens, i.e., the lens to be used, but also upon the intercept distance. For givenconditions it is thus not possible to arbitrarily choose a lens from themost appropriate of the series such as are depicted in Fig. 7. Theultimate strength and intercept distance of the lens to be chosen willdetermine which of the series 1, 2, 4, 7. diopter or other definite one,intermediate or otherwise, of such a series is suitable. From thisappropriate series a cambrure, or strength of the first surface, suchthat optimum focal surface, curvatures are secured, may be selected. 1

Thus in Fig. 1 it was indicated that a flat tangential focal surfacecould be a'desideratum.

Referring to Fig. 6 such lenses, for 25 mm. intercepts, have frontsurface strengths ranging from 13 to 16 diopters. The 7 diopter lens (23mm. intercept) of Fig. 1 is about 8% stronger and thus has a frontsurface range of about 14 to 17 diopters.

If the lens were somewhat weaker in'proportion to its intercept distanceit would be possible to choose a cambrure making the tangential focalsurface flat or even convex towards the Indeed various relationshipsbetween the two surfaces are possible and may be selected to suit thebest operating conditions for the system as a whole. and possiblesolution may be one in which both Again it may be preferable to have thetangential-convex and the sagittal concave sothat the mean curvature isapproximately zero.- Again it is obvious that for certain purposesfreedom from astigmatism is more important than minimum- Thus in someinstances a desirable curvature of field. Then the cambrure corre- 81Fig. 5).

sponding to equal focal surfaces may be chosen, (Fig. 4).

It is particularly important that, in optical systems of the typediscussed, the refracted pencils are composed of substantially parallelrays on the same side of the correcting lens as the intercept. This factdetermines a peculiar type of correcting lens and as I have discoveredmakes possibleanastigmatic corrections over a considerable strengthrange, I have discovered that the fact that these lenses are to be usedwith optical systems whose image fields are often not in themselves fiatpermits and makes desirable the choice of correcting lenses with fieldscurved so as to partly or largely neutralize the existing curvatures offield due to other elements of the system and thus improve thedefinition for that reason alone. Such lenses will in general, as thecircumstances require, have cam- -brures greater than that of the lowercambrured anastigmatic lens. They can be suitably chosen byinterpolation from the curves of Figs. 6 and '7.

If the correcting lens has much less cambrure than the smallest cambruregiving an anastigmatic solution the curvatures of the focal surfacesbecome objectionably large. When the cambrure is equal to the strengthof the lens, that is when the back surface of the correcting lens isfiat, this curvature is so large as to introduce an impracticably largeamount of refractive error into the system for oblique rays (see If theback surface of the correcting lens be convex this error is evengreater.

As will be seen from Fig. 4, the cambrure giving a definite solutionvaries with the thickness of the lens. In transposing from data lensesof the specification to definitive lenses the thicknesses are alsosubject to multiplication by the transposition factor and must beselected with this modification in mind. The curves of Figures 6 and '7are shifted with respect to the diopters first surface axis with changesin thickness, the displacements being such as to move the anastigmaticpoints in the manner indicated by the data of Fig. 4. The curves asdrawn correspond to 4 mm. lens for Fig. 6, and 3.5 mm., 3 mm. and 2 mm.lenses for the 4, 2, and 1 diopter curves of Fig. 7.

All of the data in this specification refer to a mean index ofrefractionequal to 1.52. This is an index commonly in use for simple lenses. Thedata applies equally qualitatively for other indices of refraction butinvolves a shift of the curves with respect to the diopteric firstsurface axis, namely upward for increasing index. The amount of theshift of the lower anastigmatic point is about 0.2 diopter for each 0.01of index. The distance along the diopters first surface axis between thetwo anastigmatic points (Figs. 6 and '7) is at the same time increasedby increasing the index. The shift of the upper anastigmatic point isabout 0.35 diopter for each 0.01 of index.

It will be seen from a study of the conditions in Figs. 1, 2 and 3 thatthe best intercept distance to choose in picking a lens from the graphof Fig. 4 or by interpolation from the graphs of Figs. 6 and 7 willdepend on the conditions of the optical system. The intercept distanceto be used will depend upon whether the angle is markedly greater in onemeridian or another (active or inactive plane). In some cases it will bedesirable to select as a basis for determination of In some instancesthe intercept distance in projection systems may be determined by thenature of the cone of light from the condenser. The point of the causticof the projection cone may for instance be behind, or more likely infront of the projection lens and thus in the latter case, decrease theintercept distance because the stop of the system is shifted to thefront by the nature of the caustic of the projection beam. In any casecorrecting lenses for anamorphosing systems should have surfaces concavetoward the anamorphoser. I

By interpretation of Figs. 6 and 7 it will be seen that for lenses ofthe strength and intercept distance diagrammed optimum freedom fromastigmatism is obtained by selecting the first surface strength asapproximately '7 to 9 diopters or 18 to 20, diopters representing thelesser and greater anastigmatic cambrures. The optimum flatness of fieldmaybe obtained by selecting the first surface curvature appro'ximatelywithin the range of 5 to 20 diopters and more especially within therange 3 to 18 diopters.

As indicated in the discussion of Fig. 4, if the intercept distance isgreater than 25 millimeters the particular diagrams should be read asrelating to correspondingly weaker lenses and the strength of the frontsurface should be read as correspondingly less. For convenience We maysay, for example, that the strength of the front surface should bewithin the range 5 to 20 diopters multiplied by the ratio 25 divided bythe actual intercept distance which is the equivalent of 125 to 500divided by the intercept distance.

For convenience of description in the claims, 11.3 the distance from thefront face of the correcting lens to the object in case the objective isused for photographic purposes or of the image in case the objective isused for projection will be defined as the object distance. 1

The foregoing particular description is illustrative merely and is notintended as defining the limits of the invention.

This application is a continuation in part of my application for LettersPatent, Serial No. 382,681, filed August 1, 1929. It represents in partcertain improvements over the arrangements described in thatapplication.

I claim:

1. An optical system for photographic or projection purposes comprising,in combination, a photographic lens or real image forming system, ananamorphosing unit in front of the photographic lens, and in front ofthe anamorphosing 'unit a single element correcting lens with a focallength approximating the object distance having both surfacesconcavetoward the anamorphoser.

2. An optical system for photographic or pro jection purposescomprising, in combination, a photographic lens, an anamorphosing unitin 135 front of the photographic lens, and in front of the anamorphosingunit an achromatized correcting lens with a focal length approximatingthe object distance, having its free surfaces concave toward theanamorphoser.

3. An optical system for photographic or projection purposes comprising,in combination, a photographic lens or real image forming system, ananamorphosing unit in front of the photographic lens, and in front ofthe anamorphosing 145 unit a correcting lens, correctedfor astigmatismof oblique rays, with its free' surfaces concave toward theanamorphosing unit and having a focal length approximating the objectdistance.

4. An optical system for photographic or pro- 150 front of thephotographic lens, and in front of the anamorphosing unit a singleelement correcting spherical lens having a focal length approximatingthe object distance, the radius of curvature of.

the front surface of the correcting lens being substantially two tofour. times the distance from the correcting lens to the photographicobjective thereby giving the correcting lens a cambrure such that thecorrecting lens is approximately anastigmatic for marginal inclinedrays.

5. An optical system comprising, in combination, a real image formingsystem, an anamorphosing unit in front of thissystem, and in front ofthis anamorphoser a correcting lens with a focal lengthapproxim'atingthe object distance and having a radius of curvature ofthe front surface approximately equal to 2.3 times the distance fromthelens to .the point on its axis where the principal ray of a bundle ofinclined rays incident on the front surface of the lens cuts this axisafter refraction by the lens.

6. An optical system comprising, in combination, a real image formingsystem, an anamorphosing unit in front of this system, and in front ofthis anamorphoser a correcting lens with a focal length approximatingthe object distance and having a radius of curvature of the frontsurface approximating two and a half to three times the distance fromthe lens to the point on its axis where the principal ray of a bundle ofinclined rays incident on the front surface of the lens cuts this axisafter refraction by the lens.

7. Ari-optical system for photographic or projection purposescomprising, in combination, a. photographic lens,, an anamorphosing unitin front of the photographic lens, and in front of the anamorphosingunit a correcting spherical lenswith its free surfaces concave towardthe anamorphoser, having a focal length approximating the objectdistance, the radius of curvature of the front surface of the correctinglens being less than one-half the object distance.

8. An optical system for photographic or projection purposes comprising,in combination, a photographic lens, an anamorphosing unit in front ofthe photographiclens, and in front of the anamorphosing unit anachromatized correcting lens with only two free surfaces and with afocal .length approximating the object distance and having front andback surfaces concave toward the anamorphoser, the radius of curvatureof the front surface of the correcting lens being less than one-half theobject distance.

9. An optical system for photographic or projection purposes comprising,in combination, a. photographic lens or real image forming system, ananamorphosing unit in frontof the photographic lens, and in front of theanamorphosing unit a single element correcting lens with a concave backsurface having a focal length approximating the object distance andhaving a front surface strength equal to 125 to 500 diopters di vided bythe number representing the length in millimeters of the interceptdistance.

10. An optical system for photographic or projection purposescomprising, in combination, a photographiclens or real image formingsystem, an anamorphosing unit in front of the photographic lens, and infront of the anamorphosing unit a single element correcting lenswith aconcave back surface having a focal length approximating the objectdistance and having a front surface strength equal to 225 to 450diopters divided by the number representing the length in millimeters ofthe intercept distance.-

11. An optical system for photographic orprojection purposes comprising,in combination, a

photographic lens, an anamorphosing unit in front of the photographiclens, and in front of the anamorphosing unit a single element correctinglens having a focal length approximating the object distance and havingboth faces concave toward the anamorphoser with the front surface of astrength approximately equal to 100 to 225 diopters divided by thenumber representing the length in millimeters of the intercept distance.

12. An optical system for photographic or projection purposescomprising, in combination, a photographic lens or real image formingsystem, an anamorphosing unit in front of the photographic lens, and infront of the anamorphosing unit an achromatized correcting lens with afocal length approximating the object distance, having only two freesurfaces, both surfaces being concave toward the anamorphoser and havinga front surface strength equal to 125 to 500 iopters divided by thenumber representing the length in millimeters of the intercept distance.a 13. An opticalsystem for photographic or projection purposescomprising, in combination, a

photographic lens or realimage forming system,

ing concave toward the anamorphoser and hav-" ing a front surfacestrength equal to 225 to 450 diopters divided by the number representingthe length in millimeters of theintercept distance. 14. An opticalsystem for photographic or projection purposes comprising, incombination, a photographic lens, an anamorphosing unit in front of thephotographic lens, and in front of the anamorphosing unit anachromatized correcting lens witha focal length approximating the objectdistance, having only two free surfaces, both surfaces being concavetoward the anamorphoser, and having a front surface of a strengthapproximately equal to 100 to 200 diopters divided by the numberrepresenting the length in millimeters of the intercept distance.

15. An opticalsystem for photographic or pro-,

graphic lens, and in front of the anamorphosing unit a correcting lenswith a focal length approximating the object distance and having bothfront and back surfaces concave toward the anamorphoser, the frontsurface of the lens having a strength in diopters within the range 7 to1'7 multiplied by 25 and divided by the intercept distance expressed inmillimeters.

16. An optical system comprising, in combination, a real image'formingsystem and in front thereof a single element correcting lenswith both Iand having a front surface with a strength in to the strength of thelens expressed in diopters times the intercept distance over 25, and thenumber resulting from the quotient of minus one times the thickness ofthe modified lens expressed in millimeters divided-by the strength ofthe modified lens expressed in diopters. 7

17. An optical system comprising, in combination, a real image formingsystem and-in front thereof an achromatized correcting lens with onlytwo free surfaces and with a focal length approximating the objectdistance, having front and back surfaces concave toward the system andhaving a front surface with a strength in diopters approximating theproduct of the fraction, 25 divided by the intercept distance of thelens expressed in millimeters, and the algebraic sum of the number 8,the number designating one fourth the strength of a modified lens equalto the strength of the lens expressed in diopters times the interceptdistance over 25, and the number resulting from the quotient of minusone times the thickness of the modified lens expressed in millimetersdivided by the strength of the modified lens expressed in diopters.

18. An optical system for photographic or projection purposescomprising, in combination, a photographic lens or real image formingsystem, an anamorphosing unit in front of the photographic lens and infront of the anamorphosing unit a correcting lens with only two freesurfaces and a focal length approximating the object distance, andhaving both front and back surfaces concave toward the anamorphoser, thefront surface of the lens having a strength in diopters approximatingthe product of the fraction, 25 divided by the intercept distance of thelens expressed in millimeters, and the algebraic sum of the number 8,the number designating one-fourth the strength of a modified lens equalto the strength of the lens expressed in diopters times the interceptdistance over 25, and the number resulting from the quotient of minusone times the thickness of the modified lens expressed in millimetersdivided by the strength of the modified lens expressed in diopters. I

19. An optical system for photographic or projection purposescomprising, in combination, a photographic lens, an anamorphosing unitin front of the photographic lens,.and in front of the anamorphosingunit a correcting spherical lens with its free surfaces concave towardthe anamorphoser and having a focal length approximating the objectdistance, the front surface of the correcting lens having a strength indiopters approximately equal to between 9 and 15 multiplied by 25 anddivided by the number of millimeters in the intercept distance, therebyto cause the tangential image surface of the correcting lens to beapproximately plane.

20. An optical system for photographic or pro? jection purposescomprising, in combination, a photographic lens, an anamorphosing unitin front of the photographic lens, and in front of the anamorphosingunita correctingspherical lens with its free surfaces concave toward theanamorphoser and having a focal length approximating the objectdistance, the front surface of the correcting lens having a strength indiopters approximately equal to between 10 and 16 multiplied by 25 anddivided by the number of millimeters in the intercept distance therebyto cause the sagittal image surface of the correcting lens to beapproximately plane.

21. An optical system for photographic or projection purposescomprising, in combination, a photographic lens, an anamorphosing unitin front of the photographic lens, and in front of the anamorphosingunit a correcting spherical lens lenses being concave toward the imageforming photographic lens, an anamorphosing unit in front of thephotographic lens, and in front of the anamorphosing unit a correctingspherical lens with its free surfaces concave toward the anamorphoserand having a'focal length approximating the object distance, the frontsurface of the correcting lens having a strength in dioptersapproximately equal to between 10 and 15 multiplied by 25 and divided bythe number of millimeters in the intercept distance thereby to cause thefocal surfaces to be substantially plane but deviating from the exactplane so as substantially to neutralize the curvatures of the imagesurfaces of the balance of the optical system.

23. An optical system comprising, in combination, a real image formingsystem and in front thereof a single element correcting lens with bothsurfaces concave toward the system, having a focal length approximatingthe object distance, and having a front surface with a strength indiopters approximating the product of the fraction, 25 divided by theintercept distance of the lens expressed in millimeters, and thealgebraic sum of the number 7 the number of hundredths over and above150, in the value of the index of refraction of the glass of the lens,the number designating one fourth the strength of a modified lens equalto the strength of the lens expressed in diopters times the interceptdistance over25, and the number resulting from the quotient of minus onetimes the thickness of the modified lens expressed in millimetersdivided by the strength of the modified lens expressed in diopters.

24. A series of meniscus correcting lenses of focal lengthsapproximating object distances, for use singly in combination with areal image forming system, the back surfaces of the correcting system,the front surfaces of the correcting lenses having a strength indiopters approximating the product of the fraction, 25 divided by thedistance in millimeters from the correcting lens to its naturaldiaphragm, and the algebraic sum of the number 8, the number designatingone fourth the strength of a modified lens equal to the strength of thelens expressed in diopters times the inter-, cept distance over 25, andthe number resulting from the quotient of minus one times the thicknessof the modified lens expressed in millimeters divided by the strength ofthe modified lens expressed in diopters.

25. A meniscus correcting lens of focal length approximating the objectdistance, for use in comblnation with a real image forming system, the

expressed in diopters times the intercept distance over 25, and thenumber resulting from a quotient of minus one times the thickness of themodified lens expressed in millimeters divided by the strength of themodified lens expressed in diopters.

26. A meniscus correcting lens of focal length approximating the objectdistance, foruse in combination with a real image forming system,corrected for astigmatism-of oblique rays, with its free surfacesconcave toward the image forming system, in which the strength of thefront surface of the correcting lens, expressed in diopters, is obtainedgraphically from a graph in which the strength of the front surface isplotted against the strength, of the whole lens, both expressed indiopters, and in which when the dimensions of the lens are all reducedproportionally to the ratio of the true intercept distance to anintercept distance of 25 millimeters, then according as the thickness ofthe reduced lens is 1, 3, or 5 millimeters or a value proportionatethereto, the graph passes smoothly through thesetofcoordinatepoints(4.0,.01), (6.0,.2), (7.0,.45),

(9.53.0), (10.6,1013). (1203,12); r (4.0,.10), (60,118), (6376,20)(755,41!) (8.33,6.0) (9.2,8.0) (1035,1013) (12.0,12.0) (l3.0,12.'7) j(14013.1), (15013.15); or a set of points proportionate theretorespectively, whereinfor each coordinate point the first figure withinthe parenthesis represents the ordinate giving the strength of the frontsurface in diopters, and the second figure within the parenthesisrepresents the abscissa giving the strength of the whole lens indiopters, both for the reduced lens, substantially as described.

27. A meniscus correcting lens as in claim 26 I in which the mean indexof refraction for the d line is approximately 1.52.

28. A meniscus correcting lens as in claim 26 in which the ordinants ofthe coordinate points of the graphs are algebraically increased by 0.2times the number of hundredths over and above 152 hundredths in thevalue of the mean index of refraction for the d line.

29. An optical system as in claim'l in which the focal length of thecorrecting lens is such that its strength is not more than th diopterdifferent from that for which the focal length equals the objectdistance.

diopter different from that for 30. An optical system as in claim 7 inwhich the focal length of the correcting lens is such that its strengthis not more than th diopter different from that for which the focallength equals the object distance.

31. A meniscus correcting lens as in claim 25 in which the focal lengthof the correcting lens is such-that its strength is not more than kthwhich the focal length equals the object distance.

32. An optical system as in claim 1 in .which the focal length of thecorrecting lens is such that its strength is not more than th diopterdifferent from that forwhich the focal length equals the objectdistance.

33. An optical system as in claim 7 in which the focal length of thecorrecting lens is such that its strength is not more than A th diopterdifferent from that for which the focal length equals the objectdistance. 7

34. A meniscus correcting lens as in claim 25 in which the focal lengthof the correcting lens is such that its strength is 'not more than thdiopter different from that for which the focal length equals the objectdistance.

35. An optical system as in claim 1 in which the focal length of thecorrecting lens is not more than different from the object distance.

36. An optical system as in claim '7 in which the focal length of thecorrecting lens is not more than 20% different from the object distance.

37. A meniscus correcting lens as in claim in which the focal length ofthe correcting lens is not more than 20% different from the objectdistance. I

38. An optical system as in claim 1 in which the focal length of thecorrecting lens is not more than different from the object distance.

39. An optical system as in claim 7 in which the focal length of thecorrecting lens isnot more than 50% different from the object distance.

40. A meniscus correcting lens as in claim 25 in which the focal lengthof the correcting lens is not morethan 50% different from the objectdistance. 1

41. A meniscus correcting lens as in claim 26 togetherwith ananamorphosing unit in front of which the lens is positioned, thereby toreduce the astigmatism of oblique ra'ys passing through theanamorphosing unit, substantially as described.

42. Anoptical system for-photographic or projection-purposes comprising,in combination, a photographic lens, an anamorphosing unit in front ofthe photographic lens and in front of the anamorphosing unit anachromatized correcting spherical lens having a focal lengthapproximating the object distance, the radius of curvature of the frontsurface of the correcting lens being substantially two to four times thedistance from the correcting lens to the photographic objective therebygiving the correcting lens p. cambrure lens having a focal lengthapproximating the ob- .iect distance, the radius of curvature of thefront surface of the correcting lens being less than one half the objectdistance.

44. An optical system comprising, in combination, a real image formingsystem and in front thereof a single element correcting lens with bothsurfaces concave toward the system having a focal length approximatingthe object distance, and having a front surface with a strength indiopters approximating the product of the fraetion, 25 divided by theintercept distance of the lens expressed in millimeters, and thealgebraic sum of the number 4 and 5 times the two thirds power of thequotient of the number designating the strength of a modified lens equalto the strength ofthe lens expressed in diopters times the interceptdistance over 25 and the thickness of the modified lens expressed inmillimeters.

45. An optical system for photographic or proiection purposes'comprising,'in combination, a photographic lens or real image formingsystem, an anamorphosing unit in front of the photoraphic lensand infront of the anamorphosing unit a correcting lens with only two freesurfaces and a focal length approximating the object distance, andhaving both front and back surfaces concave toward the anamorphoser, thefront surface of the lens 'having a strength in diopters approximatingthe product of the fraction, 25 divided by the intercept distance of thelens expressed in millimeters,-and the algebraic sum of the number 4 and5 times the two thirds power of the quotient of the number designatingthe I strength of a modified lens equal to the strength of the lensexpressed in diopters times the intercept distance over 25 and thethickness of the modified lens expressed in millimeters.

46. A meniscus correcting lens of focal length approximating the objectdistance, for use in combination with a real image forming system,

the back surface of the correcting lensbeing concave toward the imageforming system, the front surface of the correcting lens having astrength in diopters approximating the product of the fraction, dividedby the distance in millimeters irom the correcting lens to its naturaldiaphragm, and the algebraic sum of the number 4 and 5 times the twothirds power of the quotient of the number designating the strength of amodified lens equal to the strength of the lens expressed in diopterstimes the intercept distance over 25 and the thickness of the modifiedlens expressed in millimeters; v

47. An optical system as in claim 45 in which the focal length of thecorrecting lens is such that its strength is not more than fith diopterdifferent from that for which the focal length equals the objectdistance.

48. An optical system as in claim 45 in which the focal length or thecorrecting lens is such that its strength is not more than /;th diopterdifierent from that for which the focal length equals the objectdistance.

49. An optical system as in claim 45 in which the focal length of thecorrecting lens is not more than 20% different from the object distance.

50. An optical system as in claim 45 in which the focal length of thecorrecting lens is not more than 50% different from the object distance.

HARRY SIDNEY NEWCOMER.

